Add FAQ about controlling number of threads for pykdtree and blas

[djhoese]

@pnuu has done a lot of work learning how to control the number of threads used by dask workers. I also asked a related question on stackoverflow here. It would be nice if a question could be added to the FAQ page of the documentation describing what users might see when looking at top with high CPU usage (more than num_dask_workers * 100%) and what environment variables can be used to control that more.